Voyager at Pluto? Alternative Histories

byPaul GilsteronMarch 1, 2018

With New Horizons in hibernation as it pushes on toward MU69, it’s worth remembering how recently our knowledge of the Kuiper Belt has developed. Gerard Kuiper did not predict the belt’s existence, though he did believe that small planets or comets should have formed in the region beyond the orbit of Neptune (he also thought they would have been cleared by gravitational interactions long ago). And I always like to mention Kenneth Edgeworth’s work in a 1943 issue of the Journal of the British Astronomical Association, discussing the likelihood of small objects in the region. We could easily be calling the area the Edgeworth/Kuiper Belt, as I occasionally do in these pages.

Which takes me back to the Voyager days. It wasn’t until 1992 that astronomers discovered 15760 Albion, the first trans-Neptunian object detected after Pluto and Charon. Back in 1980, when controllers were deciding on adjustments to the trajectory of Voyager 1, Pluto was an option, as New Horizons PI Alan Stern has pointed out. The spacecraft could have reached Pluto in the spring of 1986, not long after Voyager 2’s flyby of Uranus in January of that year. That spectacular double-header was ruled out when the Voyager team chose to study Titan instead.

Image: The Voyagers’ paths through the planetary system. What if Voyager 1 had been pointed toward Pluto? Credit: NASA.

The choice to proceed with the Titan close pass occurred at a time when we simply didn’t have any information about the extent of what would soon be called the Kuiper Belt, or realize that Pluto itself could be considered a Kuiper Belt object. Interestingly, Pluto was almost exactly the same distance from the Sun in 1986 as Neptune was when Voyager 2 flew by it in 1989. Had it been sent Pluto’s way, Voyager 1’s encounter would probably have been a success.

Voyager’s ultraviolet spectrometer, Stern tells us, was not a match for the far more sophisticated Alice instrument carried on New Horizons, and the latter also brought a dust impact detector to bear, along with more powerful radio science equipment to study atmospheric temperature and pressure, and greatly improved mapping cameras. But Voyager would have brought a magnetometer, a wider range of plasma instruments, and an ability to send back data at a rate 10 times that of New Horizons. It would also have been looking at Pluto then orbiting equator-on to the Sun, as opposed to the high-latitude illumination New Horizons dealt with in its close flyby of 2015.

‘What if’s’ are always fun, and Stern looks at another in his latest PI Perspective, asking this time whether Voyager might have been able to explore the Kuiper Belt as New Horizons is now doing. Here the answer is more definitive. Without a target list, it’s difficult to see how Voyager could have made a flyby — we knew next to nothing about objects beyond Pluto as Voyager entered the region. When 15760 Albion was discovered, Voyager 1 had all but crossed the Kuiper Belt, while Voyager 2 was deep inside it. We were pulling data from within the Kuiper Belt, but lacked the ability to locate a specific KBO for a potential encounter. Writes Stern:

…with very few known KBOs at the time, and certainly no small ones known close to Voyager’s trajectory, it would have been impossible to put together a Kuiper Belt target observing list. But even had the team been able to somehow craft such a list, Voyager’s cameras used older-technology Vidicon detectors instead of the charge-coupled devices (CCDs) that LORRI [New Horizons’ Long Range Reconnaissance Imager] uses (and are found in most digital cameras). As a result, Voyager’s imagers were not anywhere near as sensitive as those aboard New Horizons, and they could not have detected faint KBOs like the telescopic LORRI can.

Image: New Horizons is the fifth spacecraft to traverse the Kuiper Belt. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Magda Saina.

Moreover, the Hubble telescope was at that time lacking the advanced wide-field camera that would later allow it to detect KBOs as small as MU69, so we simply would have had no target to aim for. Stern goes on to point out that Voyager 1 traveled well above the plane of the Solar System, while Voyager 2 was well below it, meaning the spacecraft were outside the great bulk of the KBO population. While Pluto was a Voyager possibility, a small KBO was definitely not.

Voyager’s spectrometers, magnetometers and charged-particle detectors have given us priceless data about the heliosphere and the Kuiper Belt, but New Horizons is now studying the area more intensively, viewing many KBOs from a distance, taking Kuiper Belt plasma and dust measurements with its SWAP, PEPSSI and SDC instruments, and homing in on MU69, which it will reach on January 1, 2019. The road ahead is exciting, but it’s essential that we keep working on a New Horizons successor, a craft designed from the outset to study the local interstellar medium. What surprises will this next generation of spacecraft convey?

It certainly would be nice to have another Voyager type mission get a gravity assist from Jupiter and Saturn and maybe upgrade our knowledge of Uranus and Neptune. Maybe two of them would be nice like the Voyager one and two missions except this time they go after some of the Kuiper belt objects like Eris, Sedna etc if possible and with both the remote sensing and data processing capabilities as Voyager and New Horizons.

No, especially Jupiter swing-bys are regularly possible for outer planet missions, as Jupiter has an orbital period of just 11 years. Depending on the target, the same is true for Saturn. What is not possible however, are Voyager 2-style consecutive fly-bys at all four outer planets (aka the Grand Tour), as this requires an alignment which is only available every 176 years. What is currently also not possible are consecutive fly-bys of Uranus and Neptun for similar reasons. Uranus/Neptun-only missions (both orbiter and fly-by missions) are possible and under consideration by NASA and ESA. More details can be found in the recent Planetary Decadal Survey.

The other question that comes to mind from the “Voyager 1 to Pluto” scenario is what the effect would have been on the exploration of the Saturn system. Would we have still had the Huygens lander without the initial reconnaissance from the Titan flyby by Voyager 1?

Had Pioneer 11 been given a better camera, it might have shown conclusively that Titan was completely cloud covered when it flew through the Saturn system in 1979. Even so when you look at the images the probe did return of that moon, you can tell there is no surface to be seen.

Even if we could have seen Titan’s surface, it does not mean we would have understood what we were looking at at first. Proof of this comes from the first images returned by Cassini of Titan in 2004. Scientists were plainly baffled with what they were looking at and might have remained in a state of guessitude had there not been further and more detailed examinations later on.

Personally I would have saved the serious exploration of Titan for a more dedicated mission to that moon rather than a few flybys due to that unrelenting cloud cover.

Yes Voyager 1 did detect Titan’s surface at infrared wavelengths, but this was only learned later and they were about the same as Earth-based studies, which, while helpful, left as many mysteries as when Cassini had its first look at the moon’s face.

Another “What If…?” mission, which could have occurred (had the decision been taken) because the spacecraft was already out there, was a Pioneer 11 flyby of Uranus, and perhaps also of Neptune (see: http://www.unmannedspaceflight.com/index.php?showtopic=3165 ). Pioneer 11’s instrumentation, imaging, and data communication rate capabilities at such distances from the Sun would have limited the results, but two looks at any planet yield more knowledge than just one, and the Voyager team would have gotten some “advance encounter planning intelligence” from Pioneer 11. In addition:

Any New Horizons successor(s) *should* carry, at minimum, a Pioneer 10/11-type plaque, with suitable detail changes that are appropriate to differences in the spacecraft outline (behind the human figures) and to any difference(s) in the “arrowed route” from Earth and past the planets(s) that the spacecraft encountered. Adding lines through Uranus, Neptune, and perhaps also Jupiter (to indicate that they, like Saturn, also have rings) might also be advisable, and:

The final launch vehicle stages of such probes, which in most cases (except a Pioneer 11-type “rebound” Jupiter-Saturn trajectory) would also escape from the Solar System, should also carry such plaques, with the appropriate, scale stage outline behind the human figures (and perhaps with the “arrowed routes” of both the stage and the spacecraft being shown). Such final stage plaques could be flexible, laminated metal ones that would conform to the curved stage bodies, if desired, and also:

I first wondered if Voyager 1 could (if selected to do so) have reached Pluto in 1978 or so, when I looked at the much-reproduced-elsewhere Grand Tour mission trajectory diagram in the manual that came with my 1971-vintage Rand-McNally Earth globe (it also included the 1967 Mariner 5 Venus flyby events diagram, which has also appeared in numerous books). Noting that the Voyagers’ launch dates and Jupiter-Saturn encounter dates were essentially the same as those of the then-predicted (in 1971)–but later cancelled–Grand Tour spacecraft, it certainly looked as if Voyager 1 *could* have taken the Earth-Jupiter-Saturn-Pluto route, and sure enough…

The geometrical relationships of Pioneer 11 to Uranus and Voyager 2 during 1985-86 are summarized, with special attention to conditions during Voyager 2’s encounter with Uranus on 24 January 1986. It is shown that Pioneer 11 will be able to provide valuable observations of the solar wind, the magnetic field, and energetic particle intensity in the nearby interplanetary medium before, during, and after that encounter. All of these quantities are significant in determining the state of Uranus’ magnetosphere and fluctuations thereof. (Author)

The question is, did Pioneer 11 do remote studies of Uranus in support of the Voyager 2 flyby?

Thank you–I had forgotten about Drew’s article. I don’t know what “quality” of long-distance studies that Pioneer 11–in the trajectory it did take–was able to make. Even at Saturn, its IPP (Imaging Photo-Polarimeter) spin-scan image of Titan was taken mainly for “spirit”; in “Planetary Encounters,” Robert M. Powers wrote that the IPP was at the outer limit of its capability when that picture was taken. Pioneer 11’s asteroid-meteoroid detector could have (judging by the range of Pioneer 10’s identical one while observing asteroids in the asteroid belt) made potentially useful observations at and beyond Saturn, but it had died several years previously. Also:

Solar sails could facilitate more frequent, periodic flyby missions to the outer planets (as far back as 1977, JPL looked into using the proposed Halley mission solar sail–with a Pioneer 10/11-type spacecraft instead of the Halley rendezvous probe–for such missions), with or without Jupiter gravity assists, as needed. Today, smaller spacecraft, even powered by solar cells (like the Juno Jupiter orbiter) could fly such missions. Large-area, thin-film solar cell arrays, like those incorporated into IKAROS’s solar sail, could provide adequate power even far from the Sun (JAXA’s Trojan asteroids solar sail/ion drive-powered probe will use solar power).

The August, 1970 issue of National Geographic Magazine has a huge, beautiful cover story about the Sol system with an emphasis on the Grand Tour of the Outer Planets as conceived at the time. Incredible artwork by Ludek Pesek, including a diagram of the original flight paths, which included a flyby of Pluto for one of the two space probes.

In that same article I remember the discussion on Mars: It had been imaged by Mariners 4, 6, and 7 which left the impression that the Red Planet was a dead world both geologically and biologically. The article and the art reflected this throughout, a big contrast to the NGM article on Mars by none other than Carl Sagan in the December, 1967 issue. Just over a year later, Mariner 9 would be orbiting Mars and changing our view of our celestial neighbor dramatically. NGM reflected this both in their next articles on the Red Planet and even a huge wall map.

The August, 1970 also came with a big star map and an article on the famous total solar eclipse of March 7, 1970.

I have that “National Geographic” issue, and I’ve always wished that NASA had pushed harder for the four-spacecraft mission. (The 1972 book “Beyond Jupiter: The Worlds of Tomorrow,” by Arthur C. Clarke and Chesley Bonestell [Willy Ley died before he and Bonestell had been able to create it], also highlighted the Grand Tour mission, and it covered Mariner 9’s unexpected views of Mars, including Bonestell’s depictions of it.)

If you watch the wonderful documentary on Voyager titled The Farthest, they were lucky to get two space probes for the Grand Tour as they had to ask for funding from Richard Nixon, the man who killed off Apollo and any early chances for manned missions to Mars, among his many other sins.

“I thought the gravity assists required the right planetary alignment. I suspect a Uranus/Neptune flyby cannot use the Jupiter/Saturn assists today. We really need better propulsion systems.” So true. One can always have a Jupiter assist without a planetary alignment, but if we better technology no assist is needed.

In 2017, astronomers described using the Hubble Space Telescope to look along the Voyagers’ paths. In about 40,000 years, long after both spacecraft are no longer operational, Voyager 1 will pass within 1.6 light-years of the star Gliese 445, in the constellation Camelopardalis. Meanwhile, Voyager 2 is about 10.5 billion miles (17 billion km) from Earth. Voyager 2 will pass 1.7 light-years from the star Ross 248 in about 40,000 years.

Because of the first four deep space probes to leave the Sol system, only Pioneer 10 had one world to fly by and that was Jupiter in 1973. The rest, Pioneer 11, Voyager 1, and Voyager 2, were aimed at Saturn as part of their missions and that just happened to put these other three vessels on somewhat similar trajectories.

Although if you look at their flight paths in three dimensions, the probes are actually not going in the same direction. Voyager 1 was flung north of the ecliptic after its Saturn flyby in 1981 and Voyager 2 took a sharp turn south of the ecliptic after its Neptune flyby in 1989.

In the 2 March 1979 issue of Science 203 S. J. Peale, P. Cassen and R. T. Reynolds published their paper “Melting of Io by tidal dissipation” indicating “the dissipation of tidal energy in Jupiter’s moon Io is likely to have melted a major fraction of the mass.” The conclusion of their paper was that “consequences of a largely molten interior may be evident in pictures of Io’s surface returned by Voyager 1.”

Just three days after that, the Voyager 1 spacecraft would pass within 0.3 Jupiter radii of Io. The Jet Propulsion Laboratory navigation team’s orbit estimation program as well as the team members themselves performed flawlessly. In regards to the optical navigation component image extraction of satellite centers in Voyager pictures taken for optical navigation at Jupiter rms post fit residuals were less than 0.25 pixels. The cognizant engineer of the Optical Navigation Image Processing System was astronomer Linda Morabito.

Four days after the Voyager 1 encounter with Jupiter, after preforming image processing on a picture of Io taken by the spacecraft the day before, something anomalous emerged off the limb of Io. This historical review written by the discoverer recounts her minute-by-minute quest to identify what was a volcanic plume, the first evidence of active volcanism seen beyond Earth. Many ingredients of the account reflect historic themes in the process of scientific discovery.

Comments: 30 pages, 12 figures

Subjects: History and Philosophy of Physics (physics.hist-ph); Earth and Planetary Astrophysics (astro-ph.EP); Popular Physics (physics.pop-ph)

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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